Recently, a resin-coated aluminum alloy sheet material, which does not need an insulative resin-coating process after a formation process, is used as a material for an aluminum electrolytic capacitor case. Due to the fact that this aluminum electrolytic capacitor case material is subjected to a severe formation process including a combination of a deep drawing and an ironing, if a usual resin-coated aluminum alloy sheet material for buildings or the like is used instead, cracks, peelings and so forth occur in the resin layer thereof, and thus a sufficient insulative property can not be ensured. Also, a formation process using a volatile press oil, which does not need a solvent degreasing process, prevails in order to obey the recent regulation of solvent, and thus requires a higher formability in comparison with a conventional formation process using a high viscosity oil.
On the other hand, it is well known that addition of a solid lubricant such as wax is effective in improving a formability of a resin-coated aluminum alloy sheet material. In a case where a resin layer is formed by a coating process, when wax is added to a coating material, a part (or all) of the wax cures in the state of existing on a surface of the resin layer due to the fact that the wax generally features a smaller surface energy than that of a base resin as a main component of the resin layer. When the resin-coated aluminum alloy sheet material so produced is subjected to a press formation process, the wax existing on the surface of the resin layer acts as a lubricant, and thus a press tool can smoothly slide on the surface of the resin layer so that cracks or peelings can not easily occur in the resin layer during the press formation process.
Then, a production method for a resin-coated aluminum alloy sheet material, featuring addition of wax for an improvement of the formability, has been proposed.
For example, Patent Document 1 discloses a coated metal web featuring a superior formability to an ironing, wherein, in the coated metal web material containing inner wax for a multi-stage ironing formation, regarding wax particles observed in a direction of a cross-section of a coated layer by a transmission electron microscope, a cross-sectional area of the wax particles, existing in a depth measured from the uppermost surface of a coated layer to a distance of (100−Ya)×Z/100) before an a-th formation stage, is at least 3%: Ya % is a reduction rate of a thickness of the coated layer when it is subjected to the a-th formation stage; and Z(μm) is a thickness of the coated layer before it is subjected to the a-th formation stage.
Also, Patent Document 2 discloses a production method for producing aluminum alloy sheet material for can end, wherein a coating process for coating a metal sheet with a coating material containing wax is carried out in such a manner that a temperature of the metal sheet is controlled at 110° C. after 5 seconds from the start of the coating process, and that it is then controlled at most 180° C. after 10 seconds from the start of the coating process.
The above-mentioned methods are directed to an improvement of the formability by giving a surface of a resin layer a superior lubricant property, and by making a press tool smoothly slide thereon. On the other hand, by giving a resin layer a large stretchability, progress a followingability of the resin layer to deformation of an aluminum alloy sheet material is also effective in the improvement of the formability. As a measure to give the resin layer the large stretchability, increasing a molecular weight of a base resin material of the resin layer is included.
As an example of the improvement of the formability by controlling a molecular weight of epoxy-series resin, for example, Patent Document 3 discloses a resin-coated aluminum alloy sheet material for a capacitor case, which is characterized by the fact that a resin material of the resin layer includes an epoxy-series resin as a main component, and at least one or no less than two resin materials selected from the group consisting of a phenolic resin, an acrylic resin, a urethane resin and a urea resin, features a number average molecular weight falling within a range from 5,000 to 30,000, and contains a lubricant of 0.1 to 10 weight parts per 100 weight pats of the resin, the tensile strength of the resin layer being at least 40 N/mm2, the elongation of the resin layer being at least 2%, the thickness of the resin layer falling within a range from 3 μm to 30 μm, and that a cross-cut survival rate in a cross-cut test is at least 60% when the resin-coated aluminum alloy sheet material is rolled at a rolling ratio of 40%. (Note: in this document, it is stated that a more preferable range for the number average molecular weight is from 10,000 to 20,000.)
Also, Patent Document 4 discloses a lubricated steel sheet for cans which is excellent in a drawing/ironing formability, wherein a surface of the lubricated steel sheet, which is defined as an inner wall face of a can, is coated with a coating composition composed of an epoxy resin featuring an number average molecular weight falling within a range from 2,000 to 100,000 and an epoxy equivalent weight falling within a range from 1,500 to 50,000, and wax featuring a softening point of at least 30° C., so that a coated layer is formed on the inner wall surface in a dry-coating weight falling within a range from 10-85 mg/100 cm2, the coated layer having a coefficient of dynamic friction falling within a range from 0.03 to 0.20 at the temperature of 60° C., and a pencil hardness of at least “H” at the temperature of 60° C. In Patent Document 4, it is stated that a more preferable range for the average molecular weight is from 3,000 to 70,000, and it is specified in a sub-claim that the wax is composed of at least one kind of wax selected from the group consisting of esters of fatty acid-series wax, fluorine-series wax, polyolefin-series wax, lanolin-series wax, montan wax, microcrystalline wax and carnauba wax.    Patent Document 1: JP-2005-288980 A    Patent Document 2: JP-H11-244778 A    Patent Document 3: JP-2006-334917 A    Patent Document 4: JP-2004-314415 A